The present invention relates to a reduction ratio control for a continuously variable transmission for an automotive vehicle.
In the case of an automotive vehicle having road wheels driven by an engine via a continuously variable transmission, the transmission downshifts toward the largest reduction ratio before the vehicle comes to a halt. This makes it easy for the vehicle to restart from a standstill. However, when the vehicle suddenly comes to a halt as a result of a panic braking, the transmission fails to establish the largest reduction ratio before the vehicle comes to a halt. In order to solve this problem, JP-A No. 49-103322 claiming priority on DE-A No. P2301776.7 discloses a ratio control for a continuously variable transmission wherein during a panic braking or a rapid braking, the discharge of hydraulic fluid from a driver pulley cylinder chamber is effected through bypass fluid flow line bypassing flow restricting means provided for restraining the rate of hydraulic fluid discharged from the driver pulley cylinder chamber, and thus the downshifting is effected quickly during full and rapid braking.
DE No. 36 34 166 A1 discloses a continuously variable transmission. According to this continuously variable transmission, a stepper motor of a shift operating mechanism is controlled in a closed loop manner such that an actual pulse number indicative of an actual operating position of the stepper motor approaches a target pulse number that is determined as a function of a vehicle speed and a throttle opening degree. Let us assume that the automotive vehicle with this continuously variable transmission is running on a road having a low friction coefficient. If the driver depress the brake pedal under this running condition, the vehicle starts sliding with its braked wheels being locked. Since the wheel speed is detected and used as the vehicle speed, the vehicle speed is 0 so that the transmission effects downshifting by discharging hydraulic fluid from the driver pulley cylinder chamber. This causes not only a drop in the hydraulic fluid pressure applied to the driver pulley, but also a drop in hydraulic fluid pressure applied to the follower pulley. Thus, the tension on the V-belt considerably drops. Under this condition, if the vehicle enters a road having a relatively high friction coefficient, the braked road wheels are forced to rotate owing to the inertia of the vehicle. This forced rotation of the road wheels causes the V-belt to slip relative to the driver and follower pulleys. The occurrence of slip of the V-belt causes not only damages of the V-belt, but also damages on the driver and follower pulleys. Thus operating life of the continuously variable transmission becomes short.
An object of the present invention is to improve the continuously variable transmission discussed as above such that the V-belt is prevented from slipping particularly during quick or rapid braking.